In recent years, lithium secondary batteries are widely used as power sources for electronic devices such as portable telephones and notebook computers, or for electric cars or electric power storage. Particularly recently, there is a rapidly increasing demand for a high capacity and high power battery with a high energy density, which can be mounted in hybrid cars or electric cars.
Lithium secondary batteries are primarily composed of a positive electrode and a negative electrode, which contain materials capable of absorption and desorption of lithium, and a non-aqueous electrolyte solution containing a lithium salt and a non-aqueous solvent.
Examples of positive electrode active materials used in a positive electrode include lithium metal oxides such as LiCoO2, LiMnO2, LiNiO2, and LiFePO4.
Furthermore, as the non-aqueous electrolyte solution, solutions prepared by mixing a mixed solvent (non-aqueous solvent) of carbonates such as ethylene carbonate or propylene carbonate, with a Li electrolyte such as LiPF6, LiBF4, LiN(SO2CF3)2 or LiN(SO2CF2CF3)2, are used.
On the other hand, as the active material for a negative electrode that is used in negative electrodes, metal lithium, metal compounds (elemental metals, oxides, alloys with lithium, and the like) capable of absorption and desorption of lithium, and carbon materials are known. Particularly, lithium secondary batteries employing cokes, artificial graphite or natural graphite, which are all capable of absorption and desorption of lithium, have been put to practical use.
As an attempt for improving battery performances, for example, a technique is suggested in which a compound containing phosphorus (P) and boron (B) as constituent elements is incorporated into a non-aqueous electrolyte solution (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2010-257616).
An electrolytic capacitor using a borate compound containing a phosphorus element is also suggested (see, for example, Japanese Patent No. 2966451).